The Significant Role of Cyanate Ester in Materials Research
3,013 Words
Greg Feldman
Chemical Information Retrieval
Abstract:
Cyanate ester is a thermosetting polymer that can withstand very high temperatures, as well as maintain good thermal, mechanical and electrical properties at these temperatures. These properties make cyanate ester a very attractive choice for use in many [[#|applications]] as well as the manufacture of composite materials. This is why extensive research has been done over the past few decades to understand the mechanism of its cure, its degradation, and its response to modification by nanoparticles, fibers, and other polymers. In most cases, cyanate ester maintains its properties even after modification.
Contents:
Introduction
Characteristic properties of cyanate esters
Physical characterization of cured resins
[[#|Applications]] of cyanate ester
Degradation of cyanate esters and composites
Curing of cyanate ester
Cyanate ester composite applications and properties
Conclusion
References
Introduction:
Polycyanurate esters are a class of high-temperature thermosetting resins that have a wide variety of uses. For the past century, with emphasis on the modern age, cyanate ester has been synthesized in many different ways and has generated a lot of interest in its unique properties. Early efforts to synthesize these polymers began in 1857 by the reaction of unsubstituted phenoxides with cyanogen halides which resulted in mixtures of imidocarbonates and cyanurates. (“Fang, 62”)
The first successfully synthesized polycyanurate ester was synthesized by Stroh and Gerber in 1960. (“Fang, 63”) The first commercial process for the synthesis of cyanate ester was investigated by Bayer AG in the early 1960’s geared towards using cyanate ester as a precursor to carbamates with pesticide activity. (“Fang, 63”) In the 1980’s, cyanate ester was used as a laminating resin, but was mixed with 45-55% epoxy resin to produce a cost-effective material with good characteristics. (“Fang, 63”) In the 1970’s, interest was generated in cyanate ester’s low dielectric constant for faster signal propagation in [[#|[[#|integrated circuit]]]] technology. (“Fang, 63”) Cyanate ester low dielectric constant and high temperature resistance (approx. 250C) make it ideal for integrated circuits. (“Fang, 63”)
Since 1979, considerable research has been done in toughening CE resins via addition of soluble thermoplastics to form more mechanically stable composites. (“Fang, 64”) In present day, cyanate esters have a variety of applications including insulating high speed , high density electronic circuitry, as matrix resins for aircraft composites, geostationary broadcast satellites, radomes and antennas, and have potential as versatile adhesives as well as passive waveguides or active electro optic components for processing light signals in fiber-optic communications. (“Fang, 64”)
Characteristic properties of cyanate esters:
Compared to the fully cured cyanate ester network, the unreacted monomer has a surprisingly low melting point and viscosity, which allows for solvent-less processing. (“Fang, 65”) Cyanate ester’s( –OCN) group can react with two epoxides, which allows it to be used as a curing agent for epoxy. (“Fang, 65”) Efficient toughening of cyanate ester can be achieved through the addition of thermoplastics due to cyanate ester’s ability to from co-continuous, phase-separated morphologies during cure. (“Fang, 65”) The ring forming cyclotrimerization reaction creates a very high concentration of triazine rings throughout the polymer matrix, giving cyanate ester its high glass transition temperature. (“Fang, 66”) Cyanate ester’s toughness comes from the combination of a low cross-linking density with a high concentration of sigma bonded oxygen linkages, offering more rotational freedom. (“Fang, 66”) The symmetrical arrangement of oxygen and nitrogen atoms around the carbon atoms in cyanate ester result in an absence of strong dipoles, and could explain cyanate ester’s low dielectric loss and [[#|moisture absorption]] properties. (“Fang, 66”)
Physical characterization of cured resins:
Cyanate ester developed from bisphenols results in mechanical strength comparable to that of diepoxide thermosets. (“Fang, 94”) Single ring dicyanates develop a uniquely high combination of strength, stiffness, and toughness. (“Fang, 94”) One can increase the number of cyanate functional groups on a rigid polyphenol backbone to increase to glass transition temperature of the resin via higher crosslinking density.(“Fang, 94”) Also, cyanate ester retains nearly 50 % of its shear shear stress at 170C, compared to less than 40% for epoxy at 170C. (“Fang, 94”)
Glass transition temperatures between 250C and 290C can be observed for bisphenol dicyanates.(“Fang, 95”). Cyanate ester made from novolac can produce resins with glass transition temperatures in excess of 400C. (“Marella, 2”) Rapid weight loss does not occur until in the range of 385C-431C, these onset temperatures being 20C higher than TGA scans of bismaleimide resins. (“Fang, 95”) Char yields of cyanate ester average about 15-20% higher than for diamine epoxides.(“Fang, 96”)
Under both dry and wet conditions, the conventional electrical insulating properties of cyanate ester are excellent. (“Fang, 96”) Dielectric constants of cyanate ester can be observed in the range of 2.5 to 3.0, which is significantly lower than the dielectric constants of epoxy and bismaleimide, which fall in the ranges of 3.8-4.5, and 3.3-3.7, respectively. (“ Fang, 96”) Accordingly, this very low dielectric constant makes use of cyanate ester very attractive for applications in circuitry design and microwave transmission. (“Fang, 97”) Not only does cyanate ester naturally have a lower dielectric constant than other polymer networks such as epoxy and bismaleimide, but it has the ability to retain these properties at high temperatures. (“Fang, 97”) Ortho methylation of the bisphenol precursor results in a resin most likely with more steric hinderance that can increase the onset of hydrolysis from 200 hours to 600 hours when compared to convention BPA cyanate ester. (“Fang, 98”)
Applications of cyanate ester:
Cyanate ester homopolymers and hybrids with thermoplastics and [[#|epoxy resins]] are very useful for technologically driven applications. These composite materials, using cyanate ester as their back bone, are commonly used today as electronic substrates, dielectric coatings/adhesives, aerospace composites, and microwave-transparent composites. (“Fang, 99”) Demand for high glass transistion temperatures coupled with high signal transmission for use in high power devices make cyanate ester an attractive choice due to its good cost/performance ratio. (“Fang, 99”) Cyanate ester foams with very high service temperatures can also be created using a mixture of cyanate monomers and blowing agents. (“Fang, 99) The near microwave transparency of cyanate esters make them ideal for radar tracking systems. (“Fang, 99”)
Integrated circuit technology has increased in speed and processing power over the years, as well as the thermal and electrical demands of these devices. (“Fang, 101”) Increased glass transition temperature is needed to prevent the rupture of plating in multi-layered circuit boards. (“Fang, 101”) Furthermore, the excellent metal adhesion properties of cyanate ester make it a very good substitute for epoxies in wiring boards. (“Marella, 2”) Application of microwave theory has allowed for faster signal speed and reductions in signal propagation delay, increasing the need for a resin with a low dielectric constant for use as a circuit board substrate. (“Fang, 101”) The increasing miniaturization of integrated circuits requires a resin with a low dielectric constant in order to maintain circuit impedance values. (“ Fang, 101”) Increasing circuit density, coupled with the miniaturization of these integrated circuit boards also demands that a substrate with high thermal manageability be used. (“Fang, 101”) Film deposition or film lamination is often used to apply 25-100 micrometer thickness cyanate ester films to these circuits. (“Fang, 101”) Cyanate ester is generally toughened with thermoplastic materials before use as a dielectric film for multilayer wiring boards. (“Fang, 101”)
Cyanate ester composites have been reported as having widespread use in the aerospace industry, being used in radomes, communication satellites, and other space structures such as optical benches and reflectors.(“ Fang, 103”) Additionally, cyanate ester composites are being looked at by high performance automotive industries for applications in formula 1 and Indianapolis race cars that include monocoque shells, brake-cooling ducts, fairings, and fluid reservoirs. (“Fang, 103”) Cyanate ester’s good absorption of radar frequencies made it very attractive for use in early stealth technology. (“Fang, 103”)
Ideally, radomes should be transparent to the passage of microwaves in the frequency range of 600 MHz to 100 GHz. (“Fang, 107”) Additionally, high gain antennae, horns and focusing lenses require the same transparency to microwaves. (“Fang, 107”) Cyanate ester’s low dielectric constant gives it the transparency to microwaves that, as a coating, can be effectively applied to these different technologies. (“Fang, 107”)
Cyanate ester is becoming more of an attractive polymer matrix for use in carbon fiber composites used in earth orbit due to its low out-gassing upon deployment, resistance to ionizing radiation, resistance to microcracking, resistance to stress at high temperatures, and low dielectric loss. (“Fang, 103”) This carbon fiber reinforced cyanate ester can also be used in biomedical applications such as joint and hip prosthesis(“Fang, 103”)
Degradation of Cyanate Esters and Composites:
Military aircraft uses demand polymer composites to perform at temperatures larger than 300 degrees Celsius, as well as in harsh environmental conditions. Cyanate ester’s ability to resist degradation from most solvents and acids, as well as from ultraviolet light and other forms of radiation, makes it an attractive choice for use in military aircraft composites. (“Nair, 79”) However, while cyanate ester is more resistant to water absorption compared to epoxies, it is prone to undergo hydrolysis in the presence of excess moisture. (“Nair, 79”) The by-products of this reaction that have been observed include phenols, carbon dioxide, and cyanuric acid. (“Nair, 79”) A first-order decrease in glass transition temperature with increased exposure time was also observed, showing the apparent effects of moisture on the polymer network. (“Nair, 79”) In addition to a decrease in glass transition temperature, blistering was also observed in cyanate ester laminates at temperatures in excess of 220C. (“Nair, 79”) At temperatures below 220C, the laminate is stable, possibly because blister formation is slower than the evaporation of absorbed moisture. (“Nair, 79”) The evolution of gas pockets in these laminates is directly related to the hydrolysis of the polymer network. (“Marella, 2”)
While pure cyanate ester is resistant to chemical degradation from solvents, cyanate ester composites have been proven to be more vulnerable to solvent attack. (“Nair, 80”) Prolonged exposure to moisture showed degradation in the form of delamination of the interface between the cyanate ester and the carbon fibers, due to extensive micro-cracking of the polymer matrix from increased moisture absorption. (“Nair, 80”) The glass transition temperature drops significantly just after saturation of the polymer matrix. (“Nair, 80”)
There are many reports that discuss the thermal stability of cyanate esters and cyanate ester composites, but there is still much that is not known about the thermal degradation of cyanate esters. (“Nair, 82”) The only evidence of the mechanism of thermal degradation of cyanate ester is in the by-products of the hydrolysis reaction. (“Nair, 82”) The currently proposed mechanism involves the hydrolytic cleavage of ester linkages, accompanied by decomposition of triazine rings via both hetero and hemolytic decomposition. (“Nair, 82”) Further breakdown of cyanurate by-products produces carbon monoxide, carbon dioxide, and hydrogen. (“Nair, 82”) The blistering of cyanate ester laminates at temperature over 220C can be attributed to the evolution of carbon dioxide gas during the curing of the polymer matrix. (“Nair, 82”)
Curing of Cyanate ester
Generally, cyanate ester monomer comes as a yellow liquid in with varying viscosities depending on the monomer used. For example, PT-30, which is an oligimer, has significant viscosity and must be placed in an oven at 60C to create flow so as to make it easier to work with. (“Marella, 5”) Cyclotrimerization of cyanate ester groups requires a catalyst. The catalyst used in most cyanate ester cyclotrimerization reactions is copper (II) napthanate, due to its ability to cure the polymer matrix without interfering with the conversion of cyanate groups. (“Marella, 6”) However, dibutyl tin dilaurate can prevent hydrolysis during cure. (“Marella, 38”) Curing of cyanate ester requires a multi stage curing schedule beginning with a long low temperature period, and other higher temperature, but shorter periods of cure. The resulting glass transition temperature of the polymer matrix is dependent on the temperature of the cure schedule. (“Marella, 7”) The mechanism of cure for cyanate ester includes cyclotrimerization of cyanate groups into s-traizine rings, which creates a low density, three-dimensional morphology. (“Marella, 7”) The rate of this reaction is dependent on the catalyst , the monomer used, and the schedulingused of the cure. (“Marella, 7”)
Cyanate ester composite applications and properties:
Cyanate ester/ carbon fiber composites are very interesting to the aerospace industry, however, much is not known about the aging process of these composites. (“Chung, 425”) After being subjected to a environment that accelerates aging, changes in the viscoelastic properties of the material were observed using dynamic mechanical analysis. (“Chung, 425”) Despite the composite’s very good viscoelastic properties, the interface between the polymer and the carbon fibers quickly breaks down at elevated temperatures in the presence of moisture. (“Chung, 433”)
Carbon/glass fiber cyanate ester composites offer unique structural and thermal properties that other composites such as glass, carbon, aramid, or polyethylene fibers in a resin matrix such as polyester, vinyl ester, or phenolic ester, do not offer. (“Di Blasi, 1962”) The main advantages of this composite over other composites currently used as a protective barrier for a wide variety of applications, are its high resistance to fire and flammability, which can be attributed to the properties of the cyanate ester matrix. (“Di Blasi, 1962”) While cyanate ester can withstand elevated temperatures due to its high glass transition temperature, oxidation of the cyanate ester occurs before oxidation of any other part of the composite. (“Di Blasi, 1970”) The structural and thermal properties of the composite decrease significantly with the onset of cyanate ester oxidation, which has an activation energy of 95 kJ/mol. (“Di Blasi, 1971”)
Recently, there has been much research done in the field of nanocomposites using layered clay silicates with organic modifiers. (“ Kissounko, 2807”) Cyanate ester’s attractive properties such as high glass transition temperature, low dielectric loss, and ease of processing make it an interesting resin of choice for nanoparticle dispersion. (“Kissounko, 2807”) Dispersion of clay silicate nanoparticles in cyanate ester would have synergistic effects. The effects of the dispersion of these nanoparticles would be an increase in the the rate of polymerization, as well as an increase in the conversion of cyanate groups. (“Kissounko, 2807”) The mechanism of catalysis is associated with the moisture absorbed by nanoclay particles. The organic modifiers act as moisture transport agents to ultimately facilitate the polymerization. (“Kissounko, 2807”) This modification of cyanate ester can decrease its cure temperature and increase its cure rate two fold. (“Kissounko, 2819”)
The first observation of carbon nanotubes was in 1991, and since then extensive work has been done in characterizing their mechanical and electrical properties. (“Fang, 670”) In addition to their excellent mechanical and electrical properties, they have enormous aspect ratios, which if dispersed properly can create networks of reinforcement and electrical modification across the span of the polymer matrix and ultimately of the composite made. (“Fang, 670”) Composites needed for use in very high service temperatures that also need very good mechanical properties as well as good electrical properties, such as in the aerospace industry, can be created with the use of carbon nanotubes and cyanate ester. (“Fang, 670”)
Conclusion:
Cyanate ester has a very unique polymer matrix morphology compared with other thermosetting resins. Not only this, but in its monomer form, most cyanate esters are very easy to process due to low viscosity. Additionally, many different thermoplastics are soluble in cyanate ester, allowing it to easily be modified by other polymers. Polymer networks such as epoxies and bismaleimides don’t have the same type of triazine ring polymerization that cyanate esters do. This special type of polymerization, called cyclotrimerization, gives the resulting cured resin very interesting properties. The structure of the triazine rings makes them non dipolar, giving cyanate ester its low dielectric loss. The three dimensional polymerization of triazine rigs creates a very sound structure while still maintaining a low cross linking density, which prevents brittleness and promotes a high glass transition temperature. The single bonded oxygen linkages that form during cure give the matrix the ability to move since sigma bonds have rotational freedom. However, one of cyanate ester’s main downfalls is that while it absorbs a low amount of moisture, it contains bonds within the polymer matrix that are easily hydrolysable, making cyanate ester sensitive to moisture containing environments at high temperatures, where it is most commonly found.
Cyanate ester’s high glass transition temperature, ease of processing, ability to blend with thermoplastics, toughness, low dielectric loss, and low moisture absorption are the main reasons why so many industries are interested in cyanate ester. All of these properties make cyanate ester an ideal choice for the main component of any composite that requires a high service temperature in a harsh environment.
Our technological advances across the world demand that the materials we use for such technologies also become more advanced. For instance, integrated circuits are used in almost every industry to create more advanced equipment, and with increased processing power the materials that the integrated circuits are made out of need to withstand very high temperatures as well as not interfere with signal transmission. In addition to this, these integrated circuits can be found in some extremely harsh conditions, and they need to be functional or else the entire system will not work properly or at all. This is one of many reasons why cyanate ester research is important in the materials industry.
References:
1. Chung K. Evaluation of thermal degradation on carbon fiber/cyanate ester composites. Polym Degrad Stab. 2001;71(3):425-434. Link 2. Di Blasi C. Oxidation of a carbon/glass reinforced cyanate ester composite. Polym Degrad Stab. 2009;94(11):1962-1971. Link 3. FANG T. Polycyanate esters: Science and applications. Progress in polymer science. 1995;20(1):61-118. Link 4. Fang Z. Structure and properties of multiwalled carbon nanotubes/cyanate ester composites. Polym Eng Sci. 2006;46(5):670-679. Link 5. Kissounko DA. Understanding the role of clay silicate nanoparticles with organic modifiers in thermal curing of cyanate ester resin. European polymer journal. 2008;44(9):2807-2819. Link 6. Nair C. Cyanate ester resins, recent developments. In: Advances in polymer science. ; 2001:1-99. Link 7.Marella, Vivek V, Palmese, Giuseppe R,Drexel University.College of Engineering. (2008). An investigation on the hydrolysis of polyphenolic cyanate esters using near-IR spectroscopy.Drexel University). Link
3,013 Words
Greg Feldman
Chemical Information Retrieval
Abstract:
Cyanate ester is a thermosetting polymer that can withstand very high temperatures, as well as maintain good thermal, mechanical and electrical properties at these temperatures. These properties make cyanate ester a very attractive choice for use in many [[#|applications]] as well as the manufacture of composite materials. This is why extensive research has been done over the past few decades to understand the mechanism of its cure, its degradation, and its response to modification by nanoparticles, fibers, and other polymers. In most cases, cyanate ester maintains its properties even after modification.
Contents:
Introduction
Characteristic properties of cyanate esters
Physical characterization of cured resins
[[#|Applications]] of cyanate ester
Degradation of cyanate esters and composites
Curing of cyanate ester
Cyanate ester composite applications and properties
Conclusion
References
Introduction:
Polycyanurate esters are a class of high-temperature thermosetting resins that have a wide variety of uses. For the past century, with emphasis on the modern age, cyanate ester has been synthesized in many different ways and has generated a lot of interest in its unique properties. Early efforts to synthesize these polymers began in 1857 by the reaction of unsubstituted phenoxides with cyanogen halides which resulted in mixtures of imidocarbonates and cyanurates. (“Fang, 62”)
The first successfully synthesized polycyanurate ester was synthesized by Stroh and Gerber in 1960. (“Fang, 63”) The first commercial process for the synthesis of cyanate ester was investigated by Bayer AG in the early 1960’s geared towards using cyanate ester as a precursor to carbamates with pesticide activity. (“Fang, 63”) In the 1980’s, cyanate ester was used as a laminating resin, but was mixed with 45-55% epoxy resin to produce a cost-effective material with good characteristics. (“Fang, 63”) In the 1970’s, interest was generated in cyanate ester’s low dielectric constant for faster signal propagation in [[#|[[#|integrated circuit]]]] technology. (“Fang, 63”) Cyanate ester low dielectric constant and high temperature resistance (approx. 250C) make it ideal for integrated circuits. (“Fang, 63”)
Since 1979, considerable research has been done in toughening CE resins via addition of soluble thermoplastics to form more mechanically stable composites. (“Fang, 64”) In present day, cyanate esters have a variety of applications including insulating high speed , high density electronic circuitry, as matrix resins for aircraft composites, geostationary broadcast satellites, radomes and antennas, and have potential as versatile adhesives as well as passive waveguides or active electro optic components for processing light signals in fiber-optic communications. (“Fang, 64”)
Characteristic properties of cyanate esters:
Compared to the fully cured cyanate ester network, the unreacted monomer has a surprisingly low melting point and viscosity, which allows for solvent-less processing. (“Fang, 65”) Cyanate ester’s( –OCN) group can react with two epoxides, which allows it to be used as a curing agent for epoxy. (“Fang, 65”) Efficient toughening of cyanate ester can be achieved through the addition of thermoplastics due to cyanate ester’s ability to from co-continuous, phase-separated morphologies during cure. (“Fang, 65”) The ring forming cyclotrimerization reaction creates a very high concentration of triazine rings throughout the polymer matrix, giving cyanate ester its high glass transition temperature. (“Fang, 66”) Cyanate ester’s toughness comes from the combination of a low cross-linking density with a high concentration of sigma bonded oxygen linkages, offering more rotational freedom. (“Fang, 66”) The symmetrical arrangement of oxygen and nitrogen atoms around the carbon atoms in cyanate ester result in an absence of strong dipoles, and could explain cyanate ester’s low dielectric loss and [[#|moisture absorption]] properties. (“Fang, 66”)
Physical characterization of cured resins:
Cyanate ester developed from bisphenols results in mechanical strength comparable to that of diepoxide thermosets. (“Fang, 94”) Single ring dicyanates develop a uniquely high combination of strength, stiffness, and toughness. (“Fang, 94”) One can increase the number of cyanate functional groups on a rigid polyphenol backbone to increase to glass transition temperature of the resin via higher crosslinking density.(“Fang, 94”) Also, cyanate ester retains nearly 50 % of its shear shear stress at 170C, compared to less than 40% for epoxy at 170C. (“Fang, 94”)
Glass transition temperatures between 250C and 290C can be observed for bisphenol dicyanates.(“Fang, 95”). Cyanate ester made from novolac can produce resins with glass transition temperatures in excess of 400C. (“Marella, 2”) Rapid weight loss does not occur until in the range of 385C-431C, these onset temperatures being 20C higher than TGA scans of bismaleimide resins. (“Fang, 95”) Char yields of cyanate ester average about 15-20% higher than for diamine epoxides.(“Fang, 96”)
Under both dry and wet conditions, the conventional electrical insulating properties of cyanate ester are excellent. (“Fang, 96”) Dielectric constants of cyanate ester can be observed in the range of 2.5 to 3.0, which is significantly lower than the dielectric constants of epoxy and bismaleimide, which fall in the ranges of 3.8-4.5, and 3.3-3.7, respectively. (“ Fang, 96”) Accordingly, this very low dielectric constant makes use of cyanate ester very attractive for applications in circuitry design and microwave transmission. (“Fang, 97”) Not only does cyanate ester naturally have a lower dielectric constant than other polymer networks such as epoxy and bismaleimide, but it has the ability to retain these properties at high temperatures. (“Fang, 97”) Ortho methylation of the bisphenol precursor results in a resin most likely with more steric hinderance that can increase the onset of hydrolysis from 200 hours to 600 hours when compared to convention BPA cyanate ester. (“Fang, 98”)
Applications of cyanate ester:
Cyanate ester homopolymers and hybrids with thermoplastics and [[#|epoxy resins]] are very useful for technologically driven applications. These composite materials, using cyanate ester as their back bone, are commonly used today as electronic substrates, dielectric coatings/adhesives, aerospace composites, and microwave-transparent composites. (“Fang, 99”) Demand for high glass transistion temperatures coupled with high signal transmission for use in high power devices make cyanate ester an attractive choice due to its good cost/performance ratio. (“Fang, 99”) Cyanate ester foams with very high service temperatures can also be created using a mixture of cyanate monomers and blowing agents. (“Fang, 99) The near microwave transparency of cyanate esters make them ideal for radar tracking systems. (“Fang, 99”)
Integrated circuit technology has increased in speed and processing power over the years, as well as the thermal and electrical demands of these devices. (“Fang, 101”) Increased glass transition temperature is needed to prevent the rupture of plating in multi-layered circuit boards. (“Fang, 101”) Furthermore, the excellent metal adhesion properties of cyanate ester make it a very good substitute for epoxies in wiring boards. (“Marella, 2”) Application of microwave theory has allowed for faster signal speed and reductions in signal propagation delay, increasing the need for a resin with a low dielectric constant for use as a circuit board substrate. (“Fang, 101”) The increasing miniaturization of integrated circuits requires a resin with a low dielectric constant in order to maintain circuit impedance values. (“ Fang, 101”) Increasing circuit density, coupled with the miniaturization of these integrated circuit boards also demands that a substrate with high thermal manageability be used. (“Fang, 101”) Film deposition or film lamination is often used to apply 25-100 micrometer thickness cyanate ester films to these circuits. (“Fang, 101”) Cyanate ester is generally toughened with thermoplastic materials before use as a dielectric film for multilayer wiring boards. (“Fang, 101”)
Cyanate ester composites have been reported as having widespread use in the aerospace industry, being used in radomes, communication satellites, and other space structures such as optical benches and reflectors.(“ Fang, 103”) Additionally, cyanate ester composites are being looked at by high performance automotive industries for applications in formula 1 and Indianapolis race cars that include monocoque shells, brake-cooling ducts, fairings, and fluid reservoirs. (“Fang, 103”) Cyanate ester’s good absorption of radar frequencies made it very attractive for use in early stealth technology. (“Fang, 103”)
Ideally, radomes should be transparent to the passage of microwaves in the frequency range of 600 MHz to 100 GHz. (“Fang, 107”) Additionally, high gain antennae, horns and focusing lenses require the same transparency to microwaves. (“Fang, 107”) Cyanate ester’s low dielectric constant gives it the transparency to microwaves that, as a coating, can be effectively applied to these different technologies. (“Fang, 107”)
Cyanate ester is becoming more of an attractive polymer matrix for use in carbon fiber composites used in earth orbit due to its low out-gassing upon deployment, resistance to ionizing radiation, resistance to microcracking, resistance to stress at high temperatures, and low dielectric loss. (“Fang, 103”) This carbon fiber reinforced cyanate ester can also be used in biomedical applications such as joint and hip prosthesis(“Fang, 103”)
Degradation of Cyanate Esters and Composites:
Military aircraft uses demand polymer composites to perform at temperatures larger than 300 degrees Celsius, as well as in harsh environmental conditions. Cyanate ester’s ability to resist degradation from most solvents and acids, as well as from ultraviolet light and other forms of radiation, makes it an attractive choice for use in military aircraft composites. (“Nair, 79”) However, while cyanate ester is more resistant to water absorption compared to epoxies, it is prone to undergo hydrolysis in the presence of excess moisture. (“Nair, 79”) The by-products of this reaction that have been observed include phenols, carbon dioxide, and cyanuric acid. (“Nair, 79”) A first-order decrease in glass transition temperature with increased exposure time was also observed, showing the apparent effects of moisture on the polymer network. (“Nair, 79”) In addition to a decrease in glass transition temperature, blistering was also observed in cyanate ester laminates at temperatures in excess of 220C. (“Nair, 79”) At temperatures below 220C, the laminate is stable, possibly because blister formation is slower than the evaporation of absorbed moisture. (“Nair, 79”) The evolution of gas pockets in these laminates is directly related to the hydrolysis of the polymer network. (“Marella, 2”)
While pure cyanate ester is resistant to chemical degradation from solvents, cyanate ester composites have been proven to be more vulnerable to solvent attack. (“Nair, 80”) Prolonged exposure to moisture showed degradation in the form of delamination of the interface between the cyanate ester and the carbon fibers, due to extensive micro-cracking of the polymer matrix from increased moisture absorption. (“Nair, 80”) The glass transition temperature drops significantly just after saturation of the polymer matrix. (“Nair, 80”)
There are many reports that discuss the thermal stability of cyanate esters and cyanate ester composites, but there is still much that is not known about the thermal degradation of cyanate esters. (“Nair, 82”) The only evidence of the mechanism of thermal degradation of cyanate ester is in the by-products of the hydrolysis reaction. (“Nair, 82”) The currently proposed mechanism involves the hydrolytic cleavage of ester linkages, accompanied by decomposition of triazine rings via both hetero and hemolytic decomposition. (“Nair, 82”) Further breakdown of cyanurate by-products produces carbon monoxide, carbon dioxide, and hydrogen. (“Nair, 82”) The blistering of cyanate ester laminates at temperature over 220C can be attributed to the evolution of carbon dioxide gas during the curing of the polymer matrix. (“Nair, 82”)
Curing of Cyanate ester
Generally, cyanate ester monomer comes as a yellow liquid in with varying viscosities depending on the monomer used. For example, PT-30, which is an oligimer, has significant viscosity and must be placed in an oven at 60C to create flow so as to make it easier to work with. (“Marella, 5”) Cyclotrimerization of cyanate ester groups requires a catalyst. The catalyst used in most cyanate ester cyclotrimerization reactions is copper (II) napthanate, due to its ability to cure the polymer matrix without interfering with the conversion of cyanate groups. (“Marella, 6”) However, dibutyl tin dilaurate can prevent hydrolysis during cure. (“Marella, 38”) Curing of cyanate ester requires a multi stage curing schedule beginning with a long low temperature period, and other higher temperature, but shorter periods of cure. The resulting glass transition temperature of the polymer matrix is dependent on the temperature of the cure schedule. (“Marella, 7”) The mechanism of cure for cyanate ester includes cyclotrimerization of cyanate groups into s-traizine rings, which creates a low density, three-dimensional morphology. (“Marella, 7”) The rate of this reaction is dependent on the catalyst , the monomer used, and the schedulingused of the cure. (“Marella, 7”)
Cyanate ester/ carbon fiber composites are very interesting to the aerospace industry, however, much is not known about the aging process of these composites. (“Chung, 425”) After being subjected to a environment that accelerates aging, changes in the viscoelastic properties of the material were observed using dynamic mechanical analysis. (“Chung, 425”) Despite the composite’s very good viscoelastic properties, the interface between the polymer and the carbon fibers quickly breaks down at elevated temperatures in the presence of moisture. (“Chung, 433”)
Carbon/glass fiber cyanate ester composites offer unique structural and thermal properties that other composites such as glass, carbon, aramid, or polyethylene fibers in a resin matrix such as polyester, vinyl ester, or phenolic ester, do not offer. (“Di Blasi, 1962”) The main advantages of this composite over other composites currently used as a protective barrier for a wide variety of applications, are its high resistance to fire and flammability, which can be attributed to the properties of the cyanate ester matrix. (“Di Blasi, 1962”) While cyanate ester can withstand elevated temperatures due to its high glass transition temperature, oxidation of the cyanate ester occurs before oxidation of any other part of the composite. (“Di Blasi, 1970”) The structural and thermal properties of the composite decrease significantly with the onset of cyanate ester oxidation, which has an activation energy of 95 kJ/mol. (“Di Blasi, 1971”)
Recently, there has been much research done in the field of nanocomposites using layered clay silicates with organic modifiers. (“ Kissounko, 2807”) Cyanate ester’s attractive properties such as high glass transition temperature, low dielectric loss, and ease of processing make it an interesting resin of choice for nanoparticle dispersion. (“Kissounko, 2807”) Dispersion of clay silicate nanoparticles in cyanate ester would have synergistic effects. The effects of the dispersion of these nanoparticles would be an increase in the the rate of polymerization, as well as an increase in the conversion of cyanate groups. (“Kissounko, 2807”) The mechanism of catalysis is associated with the moisture absorbed by nanoclay particles. The organic modifiers act as moisture transport agents to ultimately facilitate the polymerization. (“Kissounko, 2807”) This modification of cyanate ester can decrease its cure temperature and increase its cure rate two fold. (“Kissounko, 2819”)
The first observation of carbon nanotubes was in 1991, and since then extensive work has been done in characterizing their mechanical and electrical properties. (“Fang, 670”) In addition to their excellent mechanical and electrical properties, they have enormous aspect ratios, which if dispersed properly can create networks of reinforcement and electrical modification across the span of the polymer matrix and ultimately of the composite made. (“Fang, 670”) Composites needed for use in very high service temperatures that also need very good mechanical properties as well as good electrical properties, such as in the aerospace industry, can be created with the use of carbon nanotubes and cyanate ester. (“Fang, 670”)
Conclusion:
Cyanate ester has a very unique polymer matrix morphology compared with other thermosetting resins. Not only this, but in its monomer form, most cyanate esters are very easy to process due to low viscosity. Additionally, many different thermoplastics are soluble in cyanate ester, allowing it to easily be modified by other polymers. Polymer networks such as epoxies and bismaleimides don’t have the same type of triazine ring polymerization that cyanate esters do. This special type of polymerization, called cyclotrimerization, gives the resulting cured resin very interesting properties. The structure of the triazine rings makes them non dipolar, giving cyanate ester its low dielectric loss. The three dimensional polymerization of triazine rigs creates a very sound structure while still maintaining a low cross linking density, which prevents brittleness and promotes a high glass transition temperature. The single bonded oxygen linkages that form during cure give the matrix the ability to move since sigma bonds have rotational freedom. However, one of cyanate ester’s main downfalls is that while it absorbs a low amount of moisture, it contains bonds within the polymer matrix that are easily hydrolysable, making cyanate ester sensitive to moisture containing environments at high temperatures, where it is most commonly found.
Cyanate ester’s high glass transition temperature, ease of processing, ability to blend with thermoplastics, toughness, low dielectric loss, and low moisture absorption are the main reasons why so many industries are interested in cyanate ester. All of these properties make cyanate ester an ideal choice for the main component of any composite that requires a high service temperature in a harsh environment.
Our technological advances across the world demand that the materials we use for such technologies also become more advanced. For instance, integrated circuits are used in almost every industry to create more advanced equipment, and with increased processing power the materials that the integrated circuits are made out of need to withstand very high temperatures as well as not interfere with signal transmission. In addition to this, these integrated circuits can be found in some extremely harsh conditions, and they need to be functional or else the entire system will not work properly or at all. This is one of many reasons why cyanate ester research is important in the materials industry.
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